J. MATER. CHEM., 1992,2(9), 983-984 983
MATERIALS CHEMISTRY COMMUNICATIONS
Difluorosilylene as a Precursor for the Chemical Vapour Deposition
of Titanium S i I ic ide
C. C. Chen," J. L. Yu," C. Y. Lee,*a C. S. Liu*" and H. T. Chiu*b
a Department
of
Chemistry, National Tsing Hua University, Hsinchu, Taiwan30043,
ROCDepartment of Chemistry, National Chiao Tung University, Hsinchu, Taiwan, 30050,
ROC
A new method for preparing thin films of titanium silicide, TiSi,, by chemical vapour deposition of difluorosilylene
and titanium tetrachloride is reported.
Keywords: Chemical vapour deposition ; Titanium silicide ; Thin film
Transition-metal silicides have potential application in very large scale integrated circuits as metal-semiconductor inter- connections and as low-resistivity gates because they are good conductors, they are chemically stable at high temperature and they are resistant to corrosion and degradation.' TiSi, has widespread application and interest because it has the lowest resistivity of the silicides., Chemical vapour deposition is a good method of forming self-aligned titanium silicide films on silicon surfaces. Traditional methods involve the use of SiH, as the silicon source and TiC1, as the metal source at 700 '3C.3
Difluorosilylene, SiF,, is known to disproportionate to SiF, and Si at 650 "C., We recently found that the best temperature range for Si deposition from SiF, was 350-400°C.5 The silicon films thus obtained were found to be amorphous by XRD. ESCA analysis showed a peak characteristic for silicon oxide in addition to the peak of silicon. The Auger electron spectroscopy depth profile showed that the oxygen content dropped below 1 atom% after 10 s of sputter time ( E , =
3 keV). The fluorine content was negligible throughout the profile. The IR spectrum also showed the complete absence of v(Si-F) bands. This would suggest a new route to the synthesis of silicide thin films under milder conditions. We are particularly interested in systems where difluorosilylene and a co-precursor have a certain chemical interaction in the gas phase before deposition takes place. Titanium tetra- chloride, was chosen as a co-precursor for titanium silicide films because it volatilizes readily and is known to react with SiF, in the gas phase.?
Chemical vapour deposition reaction of SiF, and TiC1, was carried out in a Pyrex vacuum system with the reaction zone being heated externally. Difluorosilylene was generated by the reaction of SiF, and Si at 11 50 "C (with an approximate yield of ~ O Y O ) . ~ Difluorosilylene and the vapour of TiCl, were introduced into the system and thin films of TiSi, were formed on quartz, graphite and Si slides mounted in the heated zone (450-600°C). The best ratio of partial pressure was found to The film thickness can be controlled by deposition time and temperature. For example, films 2 pm in thickness can be prepared by using the partial pressures mentioned above over 2 h at 500 "C. The high purity of the crystalline TiSi, films was displayed by matching the XRD results with a standard. Both Auger electron spectroscopy (AES) and wavelength be P ~ j c 1 4 : P s ~ F ~ + s ~ F ~ = 3 : 5 (0.093 Torr : 0.1 56 Torr)..f:
t The gas-phase reaction between TiCI, and SiF, yielded a dark
1 1 Torr = (101325/760) Pa.
purple solid, the analysis of which is underway.
dispersion spectroscopy (WDS) showed the titanium and silicon in Ti : Si ratio of 1 : 2. The AES depth profile showed that the levels of 0, C, C1 and F were negligible throughout the profile. Changes in total pressure in the range 0.105- 0.046 Torr did not have any significant effect on the composi- tion of the film. An analysis of the gaseous products by GC- MS revealed that in addition to an excess of SiF,, SiClF3 and SiCl,F2 were found in an approximate molar ratio of 4 : 1.
Using these results we may speculate a reaction pathway for the formation of the TiSi, film. The overall reaction can be written as follows:
TiC1,
+
6 SiF2+2 SiClF,+
SiCl,F,+
SiF,+
TiSi, Note that in this case difluorosilylene acts both as a silicon source [eqn. (l)] and as a halogen 'scavenger' [eqn. (2) and(311.
I :
+ SiF2-
-Ti-Si-FI
I
I I
-Ti-
+ SiCI2F2 (2)Y
I
-Ti-CI + SiF2
-
-Ti-I
I
+ SiF4 (3)
The combination of these two types of reaction results in a general reaction pathway which accounts for all the results observed experimentally.
7
I
-Si-F + SiF2
-
-Si-1
I
2 SiF2
' I I
' aiCl2F2 -Ti-Si- 'TiCI, + SiF2-
CI,Ti-SiF,I
I
-SiF4I I
/
TiSi2 (4) In eqn. (4) difluorosilylene reacts with halides to generate SiF, and SiCI,F2, which forms SiCIF, via halogen exchange:
SiC12F2
+
SiF,-+2 SiClF, ( 5 )Obviously, other pathways cannot be ruled out at this time. The driving force of forming SiF4 from SiF2 that makes the deposition process clean and facile is remarkable.
It is likely that this type of reaction could be a generally
Published on 01 January 1992. Downloaded by National Chiao Tung University on 28/04/2014 18:51:43.
984 J. MATER. CHEM., 1992, VOL. 2 useful one. We are currently exploring the scope of this
method for the preparation of thin films of other metal silicides (with other volatile metal halides). For example, thin films of tantalum silicide and tungsten silicide have been prepared by reacting SiF2 with TaCl, and WF, at 500 and 600"C, respectively. The detailed analyses of these thin films are underway.
We thank the Chinese National Science Council for financial support (NSC8 1 -0208-M-007-75).
References
1 S . P. Murarka, Silicides for VLSZ Applications, Academic Press, New York 1983, and references therein; S. Wolf and R. N. Tauber, Silicon Processing for the I/LsZ Era, vol. 1.
2 S. P. Murarka, M. H. Read, C. J. Doherty and D. B. Fraser,
J . Electrochem. Soc., 1982, 129, 293.
3 J. L. Regolini, D. Bensahel, G. Bomchil and J. Mercier, A p p l . Surf. Sci., 1989, 38, 408; G. J. Reynolds, C. B. Cooper I11 and P. J. Gaczi, J . Appl. Phys., 1989, 65, 3212.
4 M. Janai, S. Aftergood, R. B. Weil and B. Pratt, J. Electrochem. SOC., 1981, 128, 2660.
5 J. L. Yu, M. S. Dissertation, National Tsing Hua University,
R.O.C. 1991.
6 P. L. Timms, R. A. Kent, T. C. Ehlert and J. L. Margrave, J. Am.
Chem. SOC., 1965,87, 2824.
Communication 2/03200G; Received 18th June, 1992
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